Animal subjects regularly exposed to stressors ranging from exercise to shock (with recovery periods) show later resistance to behavioral suppression and resistance to neuroendocrine depletion (in brain and periphery) when subjected to later (different) stressors; immune enhancement also follows from such "toughening" procedures. For humans in challenge/stress contexts, higher acute catecholamine levels correspond with better performance and emotional stability. It is hypothesized that systematic aerobic training provides an ideal "toughening" manipulation for humans, and that such training enhances catecholamine capacity (and adrenal-cortical suppression) during episodes of mental challenge or stress. Those enhanced neuroendocrine processes mediate, in turn, the temperament benefits derived from aerobic training. While preliminary work shows such neuroendocrine "training effects" in mental challenge/stress contexts, that research has not been well controlled nor sufficiently elaborated. College roommates paid to enroll as pairs will be randomly assigned to semester-long physical education courses in aerobic or non-aerobic (control) activities. Prior to lab sessions, paired subjects will sleep similar hours, eat identical meals together, and observe food restrictions to control extraneous influences on neuroendocrines. Adrenaline, nonadrenaline, dopamine, cortisol, and the metabolite MHPG will be assayed; IgA analyses will assess immune-system effects. Urine and saliva samples will be collected at the beginning and end of the semester following relaxation, mental stress, mental challenge, and an aerobic capacity test. It is predicted that catecholamine gains (and cortisol suppression) following challenge or stress will be greater for trained aerobics subjects, compared with controls and compared with themselves prior to aerobic training; however, neuroendocrines will decline more quickly in trained subjects following challenge/stress episodes. To study neuroendocrine-temperament relationships, pre- and post-semester mood and temperament inventories will be completed.